The alkali and alkaline earth metal salts of polymers containing pendant carboxyl groups (e.g., polyacrylic acid, polymethacrylic acid) exhibit ion exchange properties. These ion exchange materials are sometimes referred to as polycarboxylate ion exchange materials. Unlike many sulfonated ion exchange polymers, which pick up calcium ions in solution, these polycarboxylate ion exchange materials have low calcium selectivity and are therefore effective as toxic metal removers in environments wherein calcium is present. These polymers are generally water soluble and in order to use these polymers as ion exchange materials in aqueous media, it is necessary to convert these water soluble polymers to water insoluble polymers. This is usually accomplished by crosslinking the polymers. The ion exchange materials made from these polymers are typically in the form of beads, powders or granules.
In general there are three prior art processes for making polycarboxylate ion exchange materials. The first of these involves a process in which polymerization and crosslinking are accomplished in a single step. In this process a polymerizable carboxylic acid monomer such as acrylic acid or methacrylic acid is mixed with a crosslinking agent containing at least two polymerizable double bonds (e.g., divinyl benzene, ethylene diacrylamide), and polymerized and crosslinked in a single step. The reaction medium is in bulk, solution or suspension form and the resulting ion exchange resin products are typically in the form of beads, granules or powders. This process is not, however, suitable for making ion exchange materials in the form of thin films or composites due to the fact that the monomers are volatile and often hazardous and as a result it is impractical and sometimes hazardous to conduct the polymerization and crosslinking reactions while the reaction medium is in the form of a thin film or a composite.
The second of these processes involves copolymerization of a mixture of several monomers (e.g., acrylic acid and ethylene) to form a linear, non-crossslinked copolymer. This is accomplished in single step. At least one of the monomers (e.g., acrylic acid) has cationic functionality and by itself forms a water soluble polymer (e.g., polyacrylic acid). However, the other ethylenic monomers (e.g., ethylene) used in making the copolymer forms water insoluble linear polymers. The resulting ion exchange copolymer resins (e.g., polyacrylic acid/polyethylene) are insoluble in water, but can be melted or dissolved in certain organic liquids. These melts or organic solutions may be used to make thin films or composites. However, a disadvantage with this process is that water cannot be used as the polymer solvent.
The third process involves introducing ion functionality as a final step, after polymerization and crosslinking. The ion exchange resin material (e.g., fibers) is produced from non-ion exchange polymer or copolymer usually from solution. These polymers include polyacrylonitrile and polymethyl metharcrylate. The resin produced from these polymers is subjected to hydrolysis which produces carboxylate cation groups in the polymer. The hydrolysis reaction requires drastic treatment steps (e.g., heating with concentrated sulfuric acid or treatment with a halo sulfonic acid) which is not desirable when making thin films or composites.
U.S. Pat. No. 2,783,212 to Schnell discloses a process for preparing a water-insoluble resin capable of exchanging cations in a liquid medium which comprises copolymerizing a mixture including a polymerizable water-soluble organic acid such as acrylic acid, methacrylic acid or vinylsulfonic acid and, a polymerizable organic compound containing at least two polymerizable double bonds (e.g., divinyl benzene, ethylene glycol dimethyl acrylate, or ethylene diacrylamide) in the presence of a polymerization catalyst and condensing the resulting copolymerizate with a compound selected form the group consisting of polyhydric alcohols, polyamines and aldehydes.
The present invention overcomes the problems of the prior art by providing a simplified process for making ion exchange materials in the form of thin films and composites using water as the polymer solvent.